This invention relates generally to the art of orthopaedic prostheses, and more particularly to the type of prosthesis which consists of a supporting metal retainer or base plate portion in conjunction with a plastic bearing portion.
Many types of joint replacement devices involve the use of more than one component. For example, a total knee replacement would involve a femoral and a tibial component. In a typical knee prosthesis, the femoral component is made of metal and the tibial component is made of a plastic, such as ultra high molecular weight polyethylene. Therefore, the contact between the two components is metal to plastic. Since the prosthesis is secured in bone by a grouting agent, such as methyl methacrylate, portions of the plastic component also serve as structural members for anchorage. The plastic material in the tibial component serves as a bearing surface, as well as a structural, load-bearing member.
Clinical experience with such knee joints revealed that in more active patients, mechanical loosening of the tibial component has occurred. Upon examination of the retrieved plastic tibial components, it was discovered that cold flow or creep of the plastic bearing surface has occurred in addition to deformation of the structural portions of the component. Such observations have led to the suspicion that the material properties of plastics may have been exceeded in certain designs.
Plastics belong to the class of viscoelastic materials. These materials tend to deform permanently in time under load, a condition known as creep or cold flow. In addition, the deformation is regional and usually limited to the area where the load is born. Such a behavior can lead to a change of the dimensions of a component which can affect the anchorage of the component in bone. Gross movement of the plastic component in the grout envelope can occur and lead to mechanical loosening of the component. Since the strength of the grouting material is relatively weak, it may also fracture.
In an effort to minimize this problem, certain types of plastic bearing prosthetic components have been reinforced by encasing the plastic in a metal retainer. Such a combination utilizes the plastic as a bearing material and the retainer for structural purposes. When any load is applied to the bearing surface, it is transmitted through the plastic to the metal retainer which distributes it more evenly over the entire prosthesis. Thus, mechanical loosening due to failure of the plastic structure member can be eliminated.
An example of a prior art metal retained tibial component is illustrated in FIGS. 1 and 2. FIG. 1 illustrates the MULTI-RADIUS.TM. tibial knee component, and FIG. 2 illustrates the supporting metal base plate for this type of tibial component. MULTI-RADIUS is used as a common law trademark of Zimmer USA, Inc. It is noted that in this style of retained knee, the plastic bearing portion is totally supported by the metal retainer. The plastic portion does not extend beyond the peripheral edge of the metal retainer.
Another type of prior art metal retained joint is described in Jean-Marie Cloutier's U.S. Pat. No. 4,207,627. The tibial component of this invention is comprised of a metal tray retainer and two plastic bearing portions which locate into corresponding portions of the metal tray.
Another prior art prosthetic device utilizing a combination of metal and plastic is shown in FIG. 3 and is more fully described in Arthur D. Steffee's U.S. Pat. No. 3,506,982. The device includes a metal anchor 3 fixed into the plastic portion 4. It is noted that in a design such as that shown in the Steffee design where the plastic lips extend substantially beyond the metal anchor, the plastic may deform locally near the anchor. In time, the configuration of the socket 5 may alter and the desired motion of the prosthesis will be lost. If the loading conditions are sufficiently severe, such as that encountered in the knee, (loads up to 3 to 4 times the body weight of the patient), eventual fracture of the plastic component may also result.